WASHINGTON (AP) — Massachusetts Democrat Edward Markey on Tuesday became the newest member of the Senate, taking the oath of office after serving 37 years on the other side of the Capitol in the House.

Markey, 67, takes the seat that John Kerry won in 1984 and held until he resigned earlier this year to become secretary of State.

Last month Markey defeated Gabriel Gomez, a 47-year-old businessman and former Navy SEAL, in a special election. He will fill out the rest of Kerry’s term, which expires in January 2015, meaning he would have to run again in November next year for a full six-year term.

Markey, the son of a milkman, was first elected to the House in 1976 at age 30. He has risen to become one of the Democrats’ leading voices on energy and communications issues, and ranked eighth in seniority in the House when he stepped down.

As a freshman member of the Senate, he becomes the junior senator from Massachusetts, behind fellow Democrat Elizabeth Warren, who last November won a close race over Republican Sen. Scott Brown. Markey, backed by visits from President Barack Obama, Vice President Joe Biden and former President Bill Clinton in the final weeks of the campaign, cruised to a 55-45 win.

William “Mo” Cowan, former chief of staff to Massachusetts Gov. Deval Patrick, served as interim senator after Kerry’s departure. The Senate lineup remains at 54 on the Democratic side, including two independents, and 46 Republicans.

The next special election will be held in October, to complete the term of the late New Jersey Democrat Frank Lautenberg. New Jersey Gov. Chris Christie appointed a Republican, Jeff Chiesa, to temporarily serve as senator until that election.

Biden delivered the oath to Markey, who was flanked by Warren and Cowan.

Also oOn hand for the swearing in were House Democratic Leader Nancy Pelosi, Patrick and Rep. Jim McGovern, D-Mass.